1. Field of the Invention
The present invention relates to a light scattering type smoke detector which senses smoke by detecting light scattered by smoke. More particularly, the invention pertains to a labyrinth structure of a light scattering type smoke detector.
2. Description of the Related Art
FIGS. 1 and 2 show a plan view and a perspective view, respectively, of a conventional smoke detector. A smoke detecting section body 2 is formed in a substantially cylindrical shape. An upper wall 8 is attached to the ceiling of the room in which the smoke detector is used. On the upper wall 8, a plurality of labyrinth members 9 are provided in a standing position. A smoke detecting chamber is formed in an area surrounded by the labyrinth members 9. In order to facilitate the inflow of smoke from the outside, and to cut off light entering from the outside, each of the labyrinth members 9 is formed so as to have a substantially chevron-like or L-like horizontal section shape. Smoke inlets formed between the respective labyrinth members 9 are covered by an insect screen 5 surrounding the labyrinth members so that insects are prevented from invading the smoke detecting chamber and scattering light.
On the upper wall 8, moreover, recess portions 10 and 11, and light shielding plates 14 are disposed in a standing position. In the recess portions 10 and 11, a light emitting device 12 and a light detecting device 13 are respectively disposed so that the optical axes of the light emitting device 12 and the light detecting device 13 intersect each other at the center of the smoke detecting chamber constituted by the labyrinth members 9. The light shielding plates 14 prevent light emitted by the light emitting device 12 from directly reaching the light detecting device 13. In each of the recess portions 10 and 11, an opening is formed so that the light detecting device 13 does not directly receive the light emitted by the light emitting device 12. In the figures, the area of the visual field of the light detecting device 13 is indicated as a hatched portion b.
The detection output of the light detecting device 13 is at a zero-point level when there is no smoke. When smoke flows into the smoke detecting chamber, light scattered by the smoke is detected by the light detecting device 13. Additionally, reference numerals 16 and 17 designate holder covers for the recess portions 10, 11, respectively.
However, in the above-described conventional smoke detector, the labyrinth members 9 are merely constructed in such a manner that smoke from the outside easily enters thereinto, that light entering from the outside is cut off, and that the light detecting device 13 does not directly receive the light emitted by the light emitting device 12. Accordingly, the light emitted by the light emitting device 12 is reflected several times by flat portions, front end faces and edges of the labyrinth members 9, so as to diffuse light in the smoke detecting chamber. In addition, since the visual field of the light detecting device 13 is relatively wide, the diffused light can reach the light detecting surface of the light detecting device 13. As a result, the zero-point level of the detection output of the light detecting device 13 has to be raised. Therefore, there exists a problem in that the S/N ratio is lowered and hence the reliability and effectiveness of the smoke detector is degraded.